(La)TeX makes overfull lines

When TeX is building a paragraph, it can make several attempts to get the line-breaking right; on each attempt it runs the same algorithm, but gives it different parameters. You can affect the way TeX's line breaking works by adjusting the parameters: this answer deals with the "tolerance" and stretchability parameters. The other vital 'parameter' is the set of hyphenations to be applied: see "my words aren't being hyphenated" (and the questions it references) for advice.

If you're getting an undesired "overfull box", what has happened is that TeX has given up: the parameters you gave it don't allow it to produce a result that doesn't overfill. In this circumstance, Knuth decided the best thing to do was to produce a warning, and to allow the user to solve the problem. (The alternative, silently to go beyond the envelope of "good taste" defined for this run of TeX, would be distasteful to any discerning typographer.) The user can almost always address the problem by rewriting the text that's provoking the problem - but that's not always possible, and in some cases it's impossible to solve the problem without adjusting the parameters. This answer discusses the approaches one might take to resolution of the problem, on the assumption that you've got the hyphenation correct.

The simplest case is where a 'small' word fails to break at the end of a line; pushing the entire word to a new line isn't going to make much difference, but it might make things just bad enough that TeX won't do it by default. In such a case on can try the LaTeX \linebreak command: it may solve the problem, and if it does, it will save an awful lot of fiddling. Otherwise, one needs to adjust parameters: to do that we need to recap the details of TeX's line breaking mechanisms.

TeX's first attempt at breaking lines is performed without even trying hyphenation: TeX sets its "tolerance" of line breaking oddities to the internal value \pretolerance, and sees what happens. If it can't get an acceptable break, TeX adds the hyphenation points allowed by the current patterns, and tries again using the internal \tolerance value. If this pass also fails, and the internal \emergencystretch value is positive, TeX will try a pass that allows \emergencystretch worth of extra stretchability to the spaces in each line.

In principle, therefore, there are three parameters (other than hyphenation) that you can change: \pretolerance, \tolerance and \emergencystretch. Both the tolerance values are simple numbers, and should be set by TeX primitive count assignment - for example

\pretolerance=150

\emergencystretch is a TeX-internal 'dimen' register, and can be set as normal for dimens in Plain TeX; in LaTeX, use \setlength - for example:

\setlength{\emergencystretch}{3em}

The choice of method has time implications - each of the passes takes time, so adding a pass (by changing \emergencystretch) is less desirable than suppressing one (by changing \pretolerance). However, it's unusual nowadays to find a computer that's slow enough that the extra passes are really troublesome.

In practice, \pretolerance is rarely used other than to manipulate the use of hyphenation; Plain TeX and LaTeX both set its value to 100. To suppress the first scan of paragraphs, set \pretolerance to -1.

\tolerance is often a good method for adjusting spacing; Plain TeX and LaTeX both set its value to 200. LaTeX's \sloppy command sets it to 9999, as does the sloppypar environment. This value is the largest available, this side of infinity, and can allow pretty poor-looking breaks (this author rarely uses \sloppy "bare", though he does occasionally use sloppypar - that way, the change of \tolerance is confined to the environment). More satisfactory is to make small changes to \tolerance, incrementally, and then to look to see how the change affects the result; very small increases can often do what's necessary. Remember that \tolerance is a paragraph parameter, so you need to ensure it's actually applied - see "ignoring paragraph parameters". LaTeX users could use an environment like:

\newenvironment{tolerant}[1]{%
  \par\tolerance=#1\relax
}{%
  \par
}

\emergencystretch is a slightly trickier customer to understand. The example above set it to 3em; the Computer Modern fonts ordinarily fit three space skips to the em, so the change would allow anything up to the equivalent of nine extra spaces in each line. In a line with lots of spaces, this could be reasonable, but with (say) only three spaces on the line, each could stretch to four times its natural width.

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=overfull